1. Field of the Invention
This invention relates to highly water-absorptive polymers having enhanced gel strength. More particularly, this invention relates to highly water-absorptive polymer compositions exhibiting enhanced strength and stability of a gel impregnated with water, especially with a body fluid such as urine, and a method for producing highly water-absorptive polymers having such excellent gel properties.
2. Background Art
In recent years, highly water-absorptive polymers have been practically used not only in the field of sanitary goods such as sanitary napkins and disposable paper diapers but also for a variety of industrial materials such as water-retaining materials, dew condensation-preventing materials freshness-preserving materials and solvent-dehydrating materials. They have been put to practical use also in the fields of afforestation, agriculture and horticulture. Thus, their application is now becoming wider.
Of these application fields, sanitary goods such as sanitary napkins, disposable paper diapers and pads for those who are suffering from incontinence have been improved in a feeling upon use due to improvements in the materials used, various types of gathers, and the like. Therefore, the time during which the sanitary goods can be comfortably used is becoming longer. Also, a case where the highly water-absorptive polymers are used under extremely severe conditions is now being increased in the fields of industrial materials, agriculture and horticulture.
A highly water-absorptive polymer takes the form of a gel when it absorbs body fluids such as urine, menstrual blood and sweat. Such a gel deteriorates and decomposes with time and loses its strength, and, at the same time, the surface and inner part of the gel become sticky. Namely, the liquid-retaining properties of the gel become worse with time. Such problems caused by this that a liquid runs out without being absorbed in a sanitary good upon use, and that a feeling upon use of a sanitary good becomes worse with time, are becoming more serious as the time during which a sanitary good is used is now becoming longer.
Reasons for the deterioration and decomposition of the gel are not fully clarified so far. It is however assumed that an extremely small amount of active impurities contained in body fluids such as urine accelerate the decomposition of a highly water-absorptive polymer gel in accordance with a single mechanism or a plurality of mechanisms. The degree of and the behavior in the decomposition of the polymer gel vary widely depending upon the state of the gel when a liquid is absorbed therein and individuals' differences in a body fluid.
Under these circumstances, there has been demanded a highly water-absorptive polymer having gel properties of excellent strength and stability with time, which is always stable to body fluids such as urine regardless of individuals' differences.
As a method for improving the strength and stability of a gel, it may be considered to increase the crosslinking density of a highly water-absorptive polymer. This method, however, entails a problem of decrease in the water absorption capacity of the polymer.
Japanese Patent Kokai Publication No. 63-118375 proposes a method in which an oxygen-containing reducing inorganic salt and/or an organic antioxidant is incorporated into a polymer; Japanese Patent Kokai Publication No. 63-153060 proposes a method in which an oxidizing agent is incorporated into a polymer; Japanese Patent Kokai Publication No. 63-127754 proposes a method in which an antioxidant is incorporated into a polymer; Japanese Patent Kokai Publication No. 63-272349 proposes a method in which a sulfur-containing reducing agent is incorporated into a polymer; Japanese Patent Kokai Publication No. 63-146964 proposes a method in which a metal chelating agent is incorporated into a polymer; Japanese Patent Kokai Publication No. 63-15266 proposes a method in which a radical chain inhibitor is incorporated into a polymer; Japanese Patent Kokai Publication No. 1-275661 proposes a method in which a phosphinic acid group- or phosphonic acid group-containing amine compound or a salt thereof is incorporated into a polymer; Japanese Patent Kokai Publication No. 64-29257 proposes a method in which a polyvalent metal oxide is incorporated into a polymer; and Japanese Patent Kokai Publications Nos. 2-255804 and 3-179008 propose a method in which a water-soluble chain transfer agent is allowed to coexist when polymerization is conducted. However, all of the above-enumerated methods have problems such that the deterioration of a gel cannot be sufficiently prevented, or that an effect on preventing the deterioration of a gel is largely dependent upon individuals' differences in body fluids. Moreover, the gel strength is hardly improved when any of the above methods is employed.
In view of the aforementioned situations in the prior art, an object of the present invention is to provide highly water-absorptive polymers which are improved both in the strength and stability of a gel after when water, in particular, a body fluid such as urine which is very different between individuals, is absorbed therein, without the water absorption capacity inherent in highly water-absorptive polymers being impaired.